The several advantages of cordless power for portable power tools and certain kitchen and domestic appliances have led to the development of a wide range of sizes of power- or battery-packs, that is, a contained group of power cells. These power cells may include nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium, or lead-acid cells, etc.
During the charging process, the battery temperature and voltage vary. For example, battery temperature increases in a nonlinear manner as the battery is charged. The battery temperature then quickly increases as the battery becomes fully charged. However, if the charging process is not stopped when the battery is fully charged, the battery could be overcharged and thus damaged by the rising temperature. Accordingly, battery temperature or battery voltage are usually monitored as indicators of the full charge condition.
Among the voltage monitoring methods, the Saar double inflection termination method described in U.S. Pat. Nos. 4,388,582 and 4,392,101, is preferred to detect a battery reaching fill charge. Other voltage monitoring methods more typically employed are (1) the minus-delta-voltage method, (2) the peak detect method, and (3) the voltage slope detect method. In the minus-delta-voltage method, a sample of the battery peak voltage is stored and compared to the most recent voltage. Termination occurs when the most recent voltage falls below a set point, usually within between 0.5% and 1.0% of the stored peak, or about 10 to 20 millivolts per cell for a NiCd battery.
The peak detect method is more modern version of the minus-delta-voltage method. Basically, the same method is used, except the set point can be set closer to the peak by using more accurate instrumentation.
The slope detect method is another voltage monitoring method. According to this method, the voltage peak B is detected by calculating the slope of the voltage curve V, or voltage change rate (dV/dt). Termination occurs when the voltage change rate is 0 or negative.
Temperature monitoring methods typically employed are (1) absolute temperature termination and (2) temperature change rate termination. Absolute temperature termination relies on the temperature rise that occurs when the battery is fully charged. Under this method, the charging process will be stopped when the battery temperature reaches a certain temperature.
The temperature change rate termination method requires monitoring the changing slope of the battery temperature, or temperature change rate (dT/dt), during the charging process. Termination occurs when the temperature change rate reaches and/or exceeds a predetermined rate. In other words, termination occurs when a trip point is reached and/or exceeded. However, selecting the appropriate trip point is problematic, especially under conditions of varying ambient temperature conditions. Accordingly, the method may cause undercharged batteries.
It is preferable to provide a charging and monitoring method that ill not result in undercharged batteries.